Abstract College student drinking is a major public health problem associated with a variety of acute and chronic consequences. While alcohol is students' drug of choice, approximately 50% of student drinkers report ALC+ use (defined as alcohol plus one or more substances) regularly and experience significantly more consequences during these occasions. Despite the prevalence of ALC+ use among college students, the mechanisms underlying this specific high-risk behavior in this population are not well understood. Novel etiological data examining the combined use of alcohol and other substances are needed in order to inform the development and adaptation of efficacious intervention efforts. The formative research proposed in this R21 will be conducted across two phases: Phase 1 is a qualitative study using focus groups to elicit students' motives to engage in ALC+ use, motives to avoid ALC+ use, and contexts where ALC+ use is most prevalent. Phase 2 will consist of a longitudinal, event-level study to assess the strength of relationships between ALC+ use and the motives to drink alcohol only, motives to engage in ALC+ use, motives to avoid ALC+ use, relative to the influence of peers and situational contexts. Toward this end, the specific aims of the study propose to: 1) conduct formative research on motives to engage in or avoid ALC+ use among college students; and 2) Examine ALC+ use as a function of the motives to engage in ALC+ use (e.g., antagonistic, synergistic, and experimental), motives to avoid ALC+ use, peer and contextual influences, amount of alcohol consumed, and moderators (e.g., sex, psychological), using a longitudinal, event-level design (15 events across 5 months). In sum, this R21 study has a high potential impact to reduce a major public health problem by gathering essential etiological data on the use of alcohol combined with other substances in order to develop and adapt intervention efforts targeting this extremely high-risk behavior.